Dead reckoning

Calculating the costs of an ongoing mass extinction

The rare and endangered species shown here are among those at risk in the current human-induced mass extinction. <strong><a href=http://www.signonsandiego.com/news/metro/images/090323animals.jpg
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The rare and endangered species shown here are among those at risk in the current human-induced mass extinction. CLICK TO ENLARGE

MASS EXTINCTIONS OF THE PAST

Cretaceous-Tertiary, 65 million years ago: Most likely caused or aggravated by a miles-wide asteroid striking what is now the Gulf of Mexico. Climate change and massive volcanic eruptions are also blamed. Sixteen percent of marine families and 18 percent of land vertebrate families died out, including dinosaurs.

Triassic-Jurassic, 205 million years ago: Probably due to massive lava floods in central Atlantic region, an event that triggered the opening of the Atlantic Ocean. Volcanism could have led to catastrophic global warming. Death toll: 22 percent of marine families; land vertebrate losses unclear, but include most non-dinosaurian archosaurs and large amphibians.

Late Devonian, 364 million years ago: Actually a series of extinction events. Temperature dropped sharply, but the specific cause is not known. Twenty-two percent of marine families disappeared; little is known about land organisms at the time.

On March 2, an asteroid discovered just three days earlier narrowly missed the Earth. Dubbed 2009 DD45, it passed within 47,000 miles of the planet – a distance only slightly more than twice the altitude of a geostationary communications satellite. The moon is five times farther away.

It was a relatively small asteroid: 100 to 130 feet in diameter, roughly the size of the comet or asteroid that flattened Russia's Tunguska River region in 1908. If 2009 DD45 had actually collided with Earth, it would not have ended life as we know it. We're doing that ourselves.

Over the past 500 million years, the Earth has endured five mass-extinction events, periods when 50 to more than 90 percent of all known species perished. The last event – the Cretaceous-Tertiary event 65 million years ago, which spelled the end of the dinosaurs – was likely instigated by the impact of an asteroid far larger than 2009 DD45. But other phenomena have been cited as possible causes of mass-extinction events, including massive volcanism and extreme climate change.

Now, most scientists agree, we're in the midst of a sixth mass extinction, this one human-induced. What remains to be seen is just how bad it will be.

“Extinction is a difficult phenomenon to measure because we are still counting and describing the number of living species on Earth,” said Mark Wilson, a professor of geology at the College of Wooster in Ohio. “We may be losing tens of thousands of species every year which we haven't even met yet.”

But there is a sense among researchers that the current mass extinction, known as the Holocene event, will be very bad indeed. In the past, the Earth invariably rebounded, different but alive, eventually refilling with new and more diverse creatures and plants. Life moved on.

Odds are, humans will, too. At this point, scientists tend to think humanity will persist in some form or fashion. As a species, we are remarkably adaptable and resourceful.

The planet, not so much. Phenomena like global climate change and habitat destruction, both powerfully propelled by modern human activities, have fundamentally changed the rules. A University of Leeds study says current emission trends may raise global temperatures by the end of the century to levels not seen in 30 million years. Humans already use about half of all available land on Earth.

“There's this idea of resilience,” said Rebecca Lewison, an assistant professor of biology at San Diego State University. “How far can ecosystems be pushed before they permanently collapse? How much ecological havoc can we effect before there can be no rebound? We don't really know what the boundaries are.”

Going rate

Extinction is a part of life, with new species inevitably replacing those less able to adapt or compete. Evolution means that more than 90 percent of all species of animal and plant that have ever lived on Earth are now extinct.

But how long should a species persist? Based on the fossil record, the average lifetime of an invertebrate species, from origination to extinction, is estimated at 5 million to 10 million years. Mammal species come and go much more quickly, usually within 1 million to 2 million years.

Scientists say the “normal background extinction rate” is one species per million per year, with maybe 10 to 25 species disappearing in a year. The current extinction rate is projected at 100 to 1,000 times higher than the normal background rate.

“It's unprecedented,” said Lewison.

The United Nations Convention on Biological Diversity estimates three species become extinct each hour. That's 72 every day; 26,280 each year.

Exact numbers are debatable, but the point is not: Much of the planet's biota is imperiled. The International Union for Conservation of Nature, which maintains a highly regarded “Red List” of endangered and threatened species, estimates more than 16,300 species of animal and plant are on the verge of extinction; more than 41,000 are threatened.

The group says one-eighth of all birds, one-third of all amphibians and half of the world's turtles are in jeopardy. Seventy percent of the world's plants are considered at risk.

Blame people, says conservation biologist Michael Soule, who asserts modern extinctions are almost entirely the result of human activity – everything from habitat destruction and the introduction of non-native species to pollution, overexploitation and disease.

For example, the United Nations estimates 32 million acres of forest are lost annually, almost half of that total consisting of forest previously undisturbed by man.

Ocean acidity is rising, the result of seawater absorbing more atmospheric carbon dioxide emitted by industry and automobiles. Increased ocean acidity blocks the ability of corals and hard-shelled marine creatures to form, and hinders the growth and reproduction rates of plankton and fish.

In central Africa, the gorilla population has declined 60 percent in the last quarter-century due to local wars, the bush-meat trade and the Ebola virus.

Remnants and recovery

The ramifications of lost biodiversity are almost impossible to overstate. Recent studies have shown that grassland ecosystems with fewer plant species generally produce less biomass (living matter) than ecosystems with more species. Less plant biomass means less atmospheric carbon dioxide is absorbed and less oxygen is produced. A global decline in vegetable biomass can change the composition of gasses in the atmosphere. It means fewer plants for herbivores to eat. Entire food chains may be disrupted.

In a recently published paper, Paul Ehrlich, the Bing professor of population studies at Stanford University, and his wife, Anne, a senior research scientist, noted that since 1993, an astounding 408 new species – of mammals alone – have been discovered.

That might seem like news to celebrate, but Ehrlich, famous for his 1968 book “The Population Bomb,” suggests it also reveals how little we know about the planet's biosphere.

“Our analysis indicates how much more varied biodiversity is than we thought and how much bigger our conservation problems are if we're going to maintain the life-support services that we need from biodiversity,” Ehrlich said.

He compares nature's biodiversity to the engineered redundancy in an airplane. The “rivet hypothesis” holds that you can lose some rivets in a plane's wing and it will continue to fly, said Ehrlich. At some point, however, the loss of just one more rivet becomes catastrophic.

“Even though you don't know the value of each rivet,” said Ehrlich, “you know it's nuttier than hell to keep removing them. There is some redundancy (in nature), but we don't know how much. And facing serious climate disruption, humanity is going to need more redundancy in the little rivets, the species and populations that run the world.”

But nothing will improve as long as human behavior and activity do not, said Barry Goldstein, a biology professor at the University of Puget Sound in Washington. “The current event will last as long as habitat loss and rapid climate change continue to occur at the present rate.”

And if mass extinction goes on long enough – events have lasted from hundreds of thousands to millions of years – what's left may consist only of “weedy survivors,” said Peter Ward, a paleontologist at the University of Washington. These are animals supremely adaptable and opportunistic, such as flies, rats, crows, coyotes and intestinal parasites.

These “recovery fauna” might be the dominant organisms on Earth for a very long time. In a 2000 paper, UC Berkeley environmental scientist James Kirchner and Duke University paleontologist Ann Weil found that the average amount of time it took for the Earth's biodiversity to regain levels prior to a mass extinction event was 10 million years. The length of time didn't vary whether an extinction event was large or small.

That's far beyond any human time scale. Modern humans have only been around for a few hundred thousand years. Ancestral hominids date back only a few million years.

A recovery period of 10 million years, said Kirchner at the time of his study, “is well past the expected life span of the human species, or even of the genus Homo.”